Methodology
Sources last checked
This site exists because ounce conversions are often published wrong. So it seems fair to show our working. Every figure below is either exact by definition, or flagged as a trade custom. Nothing is rounded before it reaches your screen. Rounding happens on the display only, and you can switch it off.
One source of truth
Every conversion factor on this site is held in one place, and every page and every language reads from it. No page carries a number of its own. Automated checks then confirm the definitions still hold: sixteen standard ounces make one pound, exactly; four hundred and eighty grains make one troy ounce, exactly. If a factor is ever altered by mistake, those checks stop the site from publishing at all, rather than letting one wrong answer through in every language at once.
Exact by definition
These are not measurements. They are definitions, fixed by international agreement. There is no margin of error in them.
| Unit | Definition | Grams | In standard oz | Authority |
|---|---|---|---|---|
| pounds | exactly 0.45359237 kg | 453.59237 | 16 | International yard and pound agreement, 1959 |
| standard ounces | exactly 1⁄16 lb = 28.349523125 g | 28.349523125 | 1 | Derived from the 1959 agreement |
| stone | exactly 14 lb = 6350.29318 g | 6350.29318 | 224 | Derived from the 1959 agreement |
| grains | exactly 64.79891 mg | 0.06479891 | 0.00229 | NIST |
| troy ounces | exactly 480 grains = 31.1034768 g | 31.1034768 | 1.09714 | Derived from the grain |
| pennyweight | exactly 24 grains = 1.55517384 g | 1.55517384 | 0.05486 | Derived from the grain |
| troy pounds | exactly 12 troy ounces = 373.2417216 g | 373.2417216 | 13.16571 | Derived from the troy ounce |
| carats | exactly 200 mg | 0.2 | 0.00705 | CGPM, 1907 |
Conventions, not standards
These units are real and widely traded. But their value is a market custom, not a defined standard, and several change by region. We name which one we mean instead of pretending there is a single answer.
| Unit | Grams | In standard oz | Note |
|---|---|---|---|
| tola | 11.6638038 | 0.4114 | A South Asian market convention equal to 3/8 of a troy ounce. Regional variants exist; this is the standard value used in the Indian and Pakistani bullion trade. |
| tael (Hong Kong) | 37.79936375 | 1.3333 | Hong Kong tael — one sixteenth of a catty. Used in the Hong Kong gold trade. |
| tael (mainland China) | 50 | 1.7637 | Mainland Chinese market tael, standardised to exactly 50 g. |
| tael (Taiwan) | 37.5 | 1.3228 | Taiwanese tael, standardised to exactly 37.5 g. |
| catty (Hong Kong) | 604.78982 | 21.3333 | The Hong Kong catty, 16 taels. It is the unit the Hong Kong tael divides from, which is why that tael has the value it does. Mainland and Taiwanese catties differ. |
| baht (Thai gold) | 15.244 | 0.5377 | The weight Thai gold is bought and sold by. A one baht gold bar is 15.244 g. Ornament gold is sometimes quoted at a slightly lower baht weight, so check which is meant. |
| momme | 3.75 | 0.1323 | Japanese, and still the standard weight for pearls worldwide. One momme is exactly 3.75 g, or one thousandth of a kan. |
| masha | 0.97198365 | 0.0343 | One twelfth of a tola, used in South Asian jewellery for smaller amounts. Like the tola it is a trade custom rather than a defined standard. |
| ratti | 0.121497956 | 0.0043 | One eighth of a masha, so one ninety-sixth of a tola. Historically a seed weight, and still used for gemstones in South Asia. Several regional variants exist. |
The tael is the clearest example. A Hong Kong tael is 37.799 g. A mainland Chinese market tael is exactly 50 g. A Taiwanese tael is exactly 37.5 g. A converter with one “tael” option is giving two thirds of its users the wrong number.
Ingredient data
Cooking weights are stored as density in grams per millilitre. The source is USDA FoodData Central. Every cup, tablespoon, and teaspoon figure on the cooking page is worked out from that density when the page is built.
We do this because “one cup” is not one measurement. A US cup is 236.588 ml. A metric cup is 250 ml. A Japanese cup is 200 ml, and a Japanese rice cup is 180.39 ml again. Storing grams-per-cup would lock one country’s kitchen into the data and break every other one. Storing density means a single number stays right everywhere.
Each entry records the FoodData Central id it came from, and every one has been checked against the USDA SR Legacy bulk dataset: the id must resolve to the food we name, and its published portion must equal the weight we store. All 37 now pass.
That check was worth running. It found three errors of our own. The id for powdered sugar pointed at a tabletop aspartame sweetener, one digit off the real entry. The id for bread flour pointed at all-purpose flour. And the bread flour weight itself was wrong: we had 127 g per cup where USDA publishes 137, an eight percent error that would have quietly thrown off any bread recipe converted here. All three are fixed, and we are recording them rather than correcting them quietly.
Sometimes how you prepare an ingredient changes its packed density. Flour above all. There we publish the spread instead of one figure. Spooned, scooped, and sifted flour differ by up to 20%. Pretending otherwise is the most common reason a converted recipe fails.
Corrections
If a number here is wrong, we want to know. We will fix it and say so. Get in touch, and see who maintains this. The figures themselves are all in the converter and the chart.